3Com 4007, Implementation Manual, Page: 246 / 684

246 C
HAPTER
14: V
IRTUAL
LAN
S
(VLAN
S
)
Key Concepts
Before you configure VLANs, review the following key concepts.
Related Standards
and Protocols
The following standards and protocols apply to the VLANs that you can
configure:
■
IEEE 802.1Q is a standard for VLANs. It aims to:
■
Define an architecture to logically partition bridged LANs and
provide services to defined user groups, independent of physical
location
■
Allow interoperability between multivendor equipment.
IEEE 802.1Q defines the bridging rules for VLANs (ingress and egress
rules, as described in detail in “Rules of VLAN Operation” later in this
chapter). The standard also specifies a tag format that embeds explicit
VLAN membership information within each frame in a 12-bit VLAN ID
(VID), that provides 4094 possible VLANs. IEEE 802.1D, which now
incorporates 802.1p, uses this same frame format but it takes
advantage of an additional 3 bits to specify the priority levels used for
Class of Service differentiation.
■
Generic Attribute Registration Protocol (GARP) — This protocol is
defined in IEEE 802.1p, which is a supplement to the IEEE 802.1D
standard. GARP is a Layer 2 transport mechanism that allows switches
and end systems to propagate information across the switching
domain.
Ignore STP
mode No Yes, in allClosed
mode Ignores the blocking Spanning Tree Protocol (STP) mode for the
ports of a designated VLAN. (One instance of STP runs on the
module, but you can disable it on a per-VLAN basis.) This mode,
only available in allClosed mode, is disabled by default. You
select (on a per-VLAN basis), which VLANs ignore STP blocked
ports. It is typically used for VLANs with router interfaces that
ignore the STP state. This mode allows routing or bridging over a
port that is blocked by STP. See “Ignore STP Mode” later in this
chapter.
Table 48 VLAN Features (continued)
Feature
Layer 2
Modules and
Switch Fabric
Module
Multilayer
Modules
Description